Particles of refractory carbides or nitrides have traditionally been produced by the so-called carbothermic reaction in which an intimate mixture of carbon and an oxide of the metallic or non-metallic element is heated in an inert atmosphere to produce a carbide, or in an atmosphere of nitrogen to produce a nitride. For example, in the production of silicon carbide an intimate mixture of carbon and silica is reacted according to the overall equation EQU SiO.sub.2 +3C.fwdarw.SiC+2CO.
The problems associated with the carbothermic reaction are illustrated by the problems associated with the production of silicon carbide. Thus, in the production of silicon carbide an intimate mixture of carbon and silica is fired in an inert atmosphere at a temperature which may be as high as 2500.degree. C., the firing being effected in an electric furnace. In this process the required stoichiometric proportion of silica to carbon may readily be achieved, that is, three moles of carbon for every mole of silica, i.e. 37.5 weight per cent of carbon and 62.5 weight per cent of silica. However, the process suffers from a problem in that it is difficult to achieve the necessary intimate contact between the carbon and the silica in order that a product of uniform composition may be produced, that is of uniform composition on a molecular scale. In particular, the particles which are produced, which are nominally silicon carbide, may be contaminated with unreated silica and/or carbon. This is the case even when very small particles of silica and carbon are used, for example silica sol and carbon black. Furthermore, in this traditional process it is also difficult to produce particles of silicon carbide having a very small size, e.g. a size of less than 1 micron.
In the production of silicon nitride by the carbothermic reaction silica is similarly reacted with carbon to reduce the silica and the reduced product is reacted with nitrogen according to the overall equation EQU 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO.
The carbothermic reaction for the production of silicon nitride suffers from the same problems which are associated with the carbothermic reaction for the production of silicon carbide.
In published Japanese Patent Application No. 60-122706 there is described a modification of the silica reduction process which is said to result in production of silicon nitride in high yield with a high content of .alpha.-Si.sub.3 N.sub.4. In this modified process a powder mixture composed of 1 part by weight of silica powder, 0.4 to 4 parts of carbon powder, and 0.005 to 1 part of silicon nitride powder is fired at 1350.degree. to 1550.degree. C. in a nonoxidising atmosphere containing nitrogen or a gaseous nitrogen compound, passed at a rate of 1.0 to 2.0 cm.sup.3 /sec over the powder mixture. The silicon nitride in the powder mixture serves to accelerate the formation of crystals of silicon nitride.
Silicon nitride may be produced by direct reaction between silicon and nitrogen according to the equation EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4.
However, this process suffers from a disadvantage in that it is generally possible to produce only coarse particles of silicon nitride.
Silicon nitride may also be produced in a gas phase process in which a silicon tetrahalide or a silane is reacted with ammonia. For example, the process may be effected by reacting silicon tetrachloride with ammonia. This process also suffers from a disadvantage in that it produces copious quantities of ammonium chloride which may lead to the presence of chloride impurity in the silicon nitride which is produced.
There are a number of known processes for the production of refractory borides and silicides of metallic or non-metallic elements, particularly processes for the production of such borides and silicides in particulate form.
For example, an oxide of the metallic or non-metallic element in particulate form may be reacted in an inert atmosphere at elevated temperature in admixture with particulate carbon and particulate boron carbide. Alternatively, a particulate mixture of boric oxide, an oxide of the metallic or non-metallic element, and carbon, or a particulate mixture of boron and the metallic or non-metallic element, may be reacted in an inert atmosphere at elevated temperature. An example of the production of a boride is provided by a process for the production of titanium boride according to the reaction scheme EQU TiO.sub.2 +B.sub.2 O.sub.3 +5C.fwdarw.TiB.sub.2 +5CO
processes suffer from a problem in that it is difficult to achieve the necessary intimate contact between the components of the particulate mixture, for example between the oxide of the metallic or non-metallic element, boric oxide, and carbon, in order to produce particles of uniform composition. Furthermore, the particles of the boride of the metallic or non-metallic element which are produced may be contaminated with unreacted metallic or non-metallic element or oxide thereof and with unreacted boron, boron carbide, or boric oxide, depending of course on the composition of the particulate mixture which is used in the production process. This is the case even when very finely divided particulate mixtures are used, and furthermore, in these processes it is difficult to produce particles of the boride of the metallic or non-metallic element having a very small size, e.g. a size of less than 1 micron.
Silicides of metallic or non-metallic elements may be produced by processes similar to those described for the production of borides except that in this case the boron, or boron carbide, or boric oxide is replaced by silicon, or silicon carbide, or silica or a silicate respectively. For example, a silicide may be produced by heating a particulate mixture of silicon and the metallic or non-metallic element in an inert atmosphere. However, such process suffer from the same problems as are associated with the production of borides of metallic or non-metallic elements.
It has been proposed to produce refractory carbides such as silicon carbide by pyrolysis of organic polymeric materials which contain the elements of the ceramic material, that is silicon and carbon in the case of silicon carbide, but which do not contain oxygen. In such a process the polymeric material is first coked to convert the organic component of the polymeric material to carbon, and the carbon and silicon are then reacted in a pyrolysis reaction. This is not the traditional carbothermic reaction in which carbon and silica are reacted. The objective of using such a polymeric material is to achieve in a coked product produced from the polymeric material a more intimate mixture of the elements of the ceramic material, such as silicon and carbon, than can be achieved, for example in the case of silicon carbide, with a mixture of silica and carbon. However, the proportion of carbon to silicon in the coked product may be very different from that theoretically required with a consequent severely adverse effect on the purity of the silicon carbide which is produced.
An early example of such a "pre-ceramic" polymeric material is provided by U.S. Pat. No. 2 697 029 in which there is described the production of a polymeric material by copolymerization of a silyl substituted monomer, e.g. trimethylsilyl styrene, and another monomer, e.g. divinyl benzene or ethyl vinyl benzene, to give a cross-linked resin, and pyrolysis of the resin to give a solid containing carbon and silicon.
Further examples of such "pre-ceramic" materials are the carbosilanes produced by the pyrolysis of dodecamethylcyclohexasilane (Yajima et al, Chem. Lett., 1975, p931) and by heating poly(dimethylsilane) in an autoclave (Yajima, 1976, Nature, v.273, p525). These carbosilanes may be melt spun to fibrous materials from which refractory silicon carbide may be produced by heating at high temperature. The reaction which is effected at high temperature is between the silicon and carbon and it is not the traditional carbothermic reaction, that is the reaction between silica and carbon, referred to previously. This process suffers from the disadvantage that the silicon carbide product is impure.
A more recent example of such a "pre-ceramic" material from which a refractory carbide may be produced is provided by Japanese Patent Publication No. 57-17412 in which there is described a process in which a halogen compound or an alkoxide of silicon, vanadium, zirconium, tantalum or tungsten is reacted with a carbohydrate and the resultant reaction product is fired. The halogen compound or alkoxide may be, for example, SiCl.sub.4, ZrOCl.sub.2, Si(OC.sub.2 H.sub.5).sub.4, Si(OC.sub.2 H.sub.5).sub.3 C.sub.2 H.sub.5, Si(OC.sub.2 H.sub.5).sub.2 (CH.sub.3).sub.2, Zr(OC.sub.4 H.sub.9).sub.4, WCl.sub.2 (OC.sub.2 H.sub.5).sub.4, and the carbohydrate may be, for example, a monosaccharide or a polysaccharide, e.g. glucose, galactose, arabinose, starch, or cellulose. The reaction may be effected in the absence of a solvent but it is preferably effected in the presence of a solvent, for example, an aromatic solvent, e.g. benzene or toluene; an aliphatic solvent, e.g. hexane, heptane or octane; or a halogenated aromatic or aliphatic solvent. A coked reaction product is produced by heating the reaction product in an inert atmosphere and the coked reaction product is fired in an inert atmosphere at a temperature in the range 700.degree. to 2700.degree. C. Prior to firing the coked reaction product may be crushed to a fine powder. Although in this publication it is stated that the reaction between the halogen compound or alkoxide and the carbohydrate may be effected in a solvent and that the solvent may be used in an amount which is sufficient to dissolve or suspend the carbohydrate we find that the carbohydrates which are disclosed are not soluble in the solvents and are only capable of being suspended therein in a particulate form with the result that the reaction does not result in production of a reaction product of uniform composition or which is in a particularly tractable form. Consequently, the refractory carbide produced from the reaction product also does not have a uniform composition. Additionally, the proportion of carbon to silica in the coked reaction product may also be very different from that theoretically required.
A recent development which is described in Thermochimica Acta, 81, (1984), 77-86, is the production of silicon carbide by the pyrolysis of rice hulls. Rice hulls consist of silica and cellulose, which yield a mixture of silica and carbon when thermally decomposed. Rice hulls have a very high surface area and this, together with the intimate contact between the carbon and silica in the thermally decomposed rice hulls, enable silicon carbide to be formed by subsequent pyrolysis at relatively low temperatures. Production may be effected in a two-step process in which rice hulls are coked by heating in the absence of air at a relatively low temperature, e.g. at 700.degree. C., in order to decompose the cellulose into amorphous carbon, and the thus coked rice hulls are heated at a high temperature, e.g. at a temperature of greater than 1500.degree. C. and in an inert or reducing atmosphere to produce silicon carbide. The presence of iron in the rice hulls accelerates the reaction, and iron may be introduced by soaking the rice hulls in ferrous sulphate solution followed by soaking in ammonia. The molar ratio of silica to carbon in the coked rice hulls is generally about 1 to 4.7, that is there is a substantial excess of carbon over the stoichiometrically required proportion of 1:3, but the presence of iron influences this proportion and it is possible to achieve a proportion nearer to that which is stoichiometrically required. However, although the production of silicon carbide from rice hulls results in a product in the form of particles and whiskers, or short fibres, it is not a method which is amenable to the production of silicon carbide in a variety of different physical forms, for example, particles, long fibres, films or coatings. There is indeed a lack of control over the physical form of the silicon carbide which is produced.
Silicon nitride may also be produced by reacting rice hulls with nitrogen at an elevated temperature. Such a process is described in U.S. Pat. No. 3,855,395, the Process comprising the steps of heating rice hulls in an oxygen-free atmosphere to a temperature within the range 1100.degree. C. to 1350.degree. C. and exposing the heated rice hulls to gaseous nitrogen until the silica in the rice hulls is changed to silicon nitride. Production may be effected in a two step process in which rice hulls are coked by heating in the absence of air at a relatively low temperature, e.g. at 700.degree. C., in order to decompose the cellulose into amorphous carbon, and the thus coked rice hulls are heated at high temperature, e.g. at a temperature of the order of 1300.degree. C. and in an atmosphere of nitrogen to produce silicon nitride. However, as is the case with the production of silicon carbide the production of silicon nitride from rice hulls results in a product in the form of particles, whiskers or short fibres, it is not a method which is amenable to the production of silicon nitride in a variety of different physical forms, for example, particles, long fibres, films or coatings. There is indeed a lack of control over the physical form of the silicon nitride which is produced.
The problems associated with these previously described processes for the production of ceramic materials may be summarized with reference to the production of a refractory carbide. Thus, the quality of the refractory carbide which is produced by these previously described processes is dependent at least in part on the composition and structure of the precursor materials from which the carbide is produced and on the processing conditions. For example, although in the production of silicon carbide from a mixture of silica and carbon by the carbothermic process there is no problem in achieving the overall ratio of silica to carbon which is required to produce silicon carbide, it is impossible to achieve the intimate contact between the silica and the carbon in the carbothermic process which is necessary in order to produce a silicon carbide product of uniform composition on a microscale, let alone on a molecular scale, and which is free from unreacted silica and/or carbon.
Where the refractory carbide is produced by pyrolysis of a reaction product, e.g. a polymeric material, which contains the elements of the carbide, such as silica and carbon, e.g. which is produced by a carbothermic reaction between silica and carbon, the elements may not be present in the proportions required for producing the ceramic material substantially free of impurities, and it may be difficult to produce the refractory carbide in the physical form required, for example in the form of small particles, fibres, films or coatings. Thus, the reaction product may be intractable and be difficult to convert into the desired physical form. Where the refractory carbide is produced by pyrolysis of rice hulls there is similarly little control over the physical form of the refractory carbide.
Ceramic materials such as refractory carbides and nitrides have been used for many years in such applications as abrasives and in the manufacture of tools. Whereas in these applications the quality of the ceramic material might not have been of critical importance there are other applications of ceramic materials which are of more recent development where the quality of the ceramic material and its physical form may be of critical importance. These more recently developed applications of ceramic materials include applications such as engineering materials and use in electronic applications.